1. Field of the Invention
The present invention relates to probe cards and more particularly, to a cantilever-type probe card for high frequency application.
2. Description of the Related Art
FIG. 1 shows a cantilever-type probe card 1 according to the prior art. According to this design, the probe card 1 comprises a circuit board 10, a plurality of coaxial transmission lines 11, a probe holder 12, and a plurality of probes 13. The circuit board 10 has a plurality of contact pads 101 on the top side near the border for receiving test signals from a test machine. The coaxial transmission lines 11 are electrically connected between the contact pads 101 and the probes 13 for transmission of test signals from the test machine to the probes 13 which can electrically probe ICs of a semiconductor wafer.
The probe holder 12 has a grounding plane 121, a holder body 122, and a plurality of fastening members 123. The grounding plane 121 is electrically connected to the ground potential of the probe card 1. The probes 13 are mounted on the probe holder 12 and each comprises a tip 130, a body fastened to each of the fastening members 123, a front part 131 located between the tip and the fastening member 123, and a rear part 132 electrically connected to a contact pad 102 on the bottom side of the circuit board 10. The front part 131 of each of the probes 13 is suspended beneath the probe holder 12 to provide a lever arm for the tip 130 to buffer the anti-force reacted from the probed IC to the tip 130 during the wafer level test.
Further, each of the coaxial transmission lines 11 has an electrically grounded metal shield 111 provided axially around an axial wire to maintain the characteristic impedance of transmission for high-frequency signal through the coaxial transmission line 11. However, because the probes 13 are simply formed of a conducting metal without having a similar design functioned like the coaxial transmission lines 11, the parasitic capacitor that is induced subject to the dielectric surrounding around each of the probes 13 may cause the dielectric loss during a high frequency signaling through the probe 13, thus lowering the reliability of the wafer level test.
FIG. 2 shows an improved structure of cantilever-type probe card 2 according to the prior art. According to this design, a plurality of coaxial probes 20 are used to substitute the aforesaid single conducting metal of the probe 13, and the holder body 122 of the probe holder 12 is made of a conducting material. Each of the coaxial probes 20 comprises a metal pin 21, which has a front part 201 defined between a respective probing tip and the associating fastening member 123 and suspended beneath the probe holder 12 and a rear part 202 set between the associating fastening member 123 and the circuit board 10, a dielectric material 22 wrapping the rear part 202 of the metal pin 21, and an outer conducting layer 23 wrapping the dielectric material 22 and electrically connected with the holder body 122 to the ground potential of the probe card 2, so that the rear part 202 of the metal pin 21 of each coaxial probe 20 can maintain the characteristic impedance of transmission for a high frequency signal. However, because the front part 201 of each coaxial probe 20 is adapted to buffer the anti-force when the probing tip is probing the test sample, it is required for keeping its specific weight and being flexible, so the front part 201 of the coaxial probe 20 cannot be designed as a coaxial structure like the rear part 202 having wrapped by the dielectric material 22 and the outer conducting layer 23 that is electrically connected to the holder body 122 of the probe holder 12. Therefore, the characteristic impedance of a high frequency signaling through each coaxial probe 20 is maintained only at the rear part 202 of the metal pin 21, not reaching the front part 201. As a result, the dielectric loss also exists subject to the parasitic capacitor induced by the dielectric surrounding around the front part 201 of the metal pin 21 during transmission of a high frequency signal through the coaxial probe 20.
Therefore, it is desirable to provide a cantilever-type probe card that maintains the characteristic impedance during high frequency signaling and prevents stress damage to both the probes and the relative probed ICs during the wafer level test.